LASERS

LASER is an acronym for light amplification by Stimulated Emission of radiation.

When radiation interacts with matter we have three processes to generate laser light.

(1) Optical Absorption(2) Spontaneous Emission(3) Stimulated Emission

Characteristics of Lasers

The most important characteristics of lasers are

1.Directionality → Angular spread 10 micro radians

2.Monochromacity

3.Coherence → (a). Spatial coherence

(b). Temporal coherence

4.Intensity:

The number of photons coming out from a laser per second per unit area is given by

2rh

Pn

Spontaneous Emission

• Incoherent • Less intensity• Poly chromatic• Less directionality• More angular spread

Stimulated emission

• Coherent • High intensity• Mono chromatic• High directionality• Less angular spread

absorption

Spontaneous

emission

Stimulated emission

E1

E2

N1

N2

)(Supplied energy

Einstein Co-efficient• Consider two energy levels E1 and E2.

• N1 and N2 be the number of atoms per unit volume present at the levels E1 and E2 respectively.

radiation. ginteractin ofdensity energy (v) & ,absorption stimulated of

t coefficienEinstien theis Bality proportion ofconstant theWhere

)(B

(v) & N toalproportion is rate absorption Stimulated

:Absorption (1)

12

112

1

vN

emission. sspontaneou of

tcoefficienEinstien theisAality proportion ofconstant the Where

)2(A

N toalproportion is rateemission sSpontaneou

:Emission sSpontaneou (2)

21

221

2

N

emission. stimulated oft coefficienEinstien theisB

ality proportion ofconstant thee Wher

)3...().........(B

)(&N toalproportion is rateemission Stimulated

:Emission Stimulated (3)

21

221

2

vN

v

For system in a equilibrium

1)(

)(

)(

)()(

)()(

21

12

2

1

21

21

21122

1

21

212121

212

212212121

212212121

BB

NNBA

v

BBNN

Av

BNBN

ANv

ANBvNBvN

ANBvNBvN

)6.....(}.........1)

kTh

exp(

1{

8(v)

bygiven isdensity radation the

radiation,body black of law splanck' from

)5.....(..........1)exp(

)(

.E &E levelsenergy of s'degeneracy are g & g Where

)exp(

lawon distributiman Bolts toAccording

3

3

2

1

21

12

21

21

2121

2

1

2

1

c

h

kTh

gg

BB

BA

v

kT

h

g

g

N

N

Comparing equations (5) & (6)

)8........(8

)7........(

1

3

3

21

21

212121

2

1

21

12

c

h

B

A

BgBg

g

g

B

B

1)kT

hexp(R

}1)

kTh

exp(

1{

8

)8

(

)(

)8

(R

8

)(

)(

3

3

3

3

3

3

21

21

212

212

ch

ch

vch

eqfrom

Bv

AR

BvN

ANR

n

The equation shows ratio of spontaneous

Emission Rate to stimulated emission rate.

Population inversion To achieve more stimulated emission population of the

excited state N2 should be made larger than the lower state N1 and this condition is called population inversion.

E1

E2

N1

N2

12 NN

Three level Laser system

N

E

E0

E1

E2 Fast decay

Laser transition

pumping

Meta stable level

Four level laser system

N

E

E0

E1

E2

Fast decay

Laser transitionpumping

E3

Meta stable state

Ruby LASER

1. Maiman in 1960.2. Solid State Laser.3. Active Medium: Al2O3 doped with 0.05% Cr3+

ions(10cm long & 0.5cm diameter).4. Resonant Cavity: Fully reflecting surface &

partially reflecting surface.5. Pumping System: Helical Xenon flash lamp.6. Three level laser system.7. Wave Length: 694.3nm.8. Pulsed Laser.9. Widely used in Echo technique & Pulsed

Holography

Partially reflecting

surface

Completely

Reflecting

surface

cooling

Xenon flash lamp

Ruby material

Partially reflecting

surface

Completely

Reflecting

surface

cooling

Xenon flash lamp

Ruby material

Laser

output

N

E

E0

E1

E2 Fast decay

Laser transition

pumping

Meta stable level

Energy level diagram of Ruby laser

Short lived state

He-Ne LASER

1.Ali Javan in 1961.2.Gas Laser..3.Active Medium: Helium & Neon Mixture

10:1 ratio...at 0.1mm of Hg.4.Resonat Cavity: Fully & partially reflected

surfaces…5.Pumping System: Discharge electrodes…6.Four level Laser System.7.Wave Length:632.8nm.8.Red color Continuous Laser.9.Widely used in Interferometer Experiments &

Holography.

Discharge

electrodes

He + Ne (10:1)

0.1mm of Hg

Discharge tube

Fully reflecting

mirrorPartially reflecting

mirror

Laser out put

N

E

E0

E1

E2

Fast decay

Laser transitionpumping

E3

Meta stable state

NeHe

Energy level diagram of He-Ne laser

CO2 LASER

1.CKN Patel in 1963.2.Gas Laser..3.Active Medium: CO2 , N2 & helium mixture

1:4:5 ratio...at 0.1mm of Hg.4.Resonat Cavity: Fully & partially

reflected surfaces…5.Pumping system: Discharge electrodes…6.Large no of energy levels are contributes

for out put laser radiation..7.Wave Length:10.6micro meters.8.Pulsed & Continuous Laser.9.Widely used in Material processing such as

Cutting , Drilling, Welding.

Fully reflected

surface

Partially reflected

surface

Cooling

Discharge electrodes

Co2+N2+He

1:4:5

Out put

laser

Energy level diagram of co2 laser

He N2

E

pumping

Fast decay

Laser

transition

collisions collisions

co2

Vibrational modes of the CO2 molecule

Oxygen OxygenCarbon

Symmetric

mode

Bending

modes

Asymmetric

modes

Semiconductor Laser

1.Semiconductor Laser is also called as Diode Laser.

2.The wave length of the emitted light depends upon the Energy band gap of the material.

3.Diode Lasers are always operated in forward bias..4.Working Principle: When we apply forward bias

to a PN-Diode, charge carrier recombination takes place.. Then in such a process the energy is emitted in the form of light radiation..

5.Active Medium: GaAlAs diode or GaAsP diode..6.Out put Wave length: GaAlAs:750-900nm,.GaAsP:1100-1600nm..7.Pulsed & Continuous Laser…

g

g

g

E

hc

hcE

hE

The Energy band gap of a material

Where c is the velocity of light & h is Planck's constant.

P

NActive

region

Positive

Metal contact

Negative

Metal contact

Forward

biasing